In a so-called SU-type variable venturi carburetor, this invention relates to a carburetor wherein a negative pressure of a mixing chamber is communicated with a suction chamber in order to cause change in the cross-section of a venturi section by means of a suction piston sliding in the suction chamber under correlationship between spring balance and leak operation gas pressure, and a metering is provided to the head of the suction piston so as to face a metering jet. More specifically, the present invention relates to a variable venturi carburetor of the type wherein the suction piston closes the venturi section at engine stop so as to elevate a negative pressure at the time of low temperature cranking and thus to ensure easy intake for the engine and enrichment of an air-fuel mixture, and wherein the gap between the metering needle and the metering jet is made greater at the time of engine stop than at the time of idling and a lift movement prior to idling is made small while a lift movement at the time of idling is made relatively large.
As is well known in the art, in a carburetor of an engine, various kinds of fixed- and variable-venturi carburetors have been developed. Due to various advantages of the variable venturi carburetor such as good response characteristics to the change in an air-fuel mixture, absence of a branching system of a slow system and a main system and so forth, the variable venturi carburetor has gradually been installed to cars at large, ranging initially from some sport cars to passenger cars in general.
However, there are various problems yet to be solved in the variable venturi carburetor. Among them is a troublesome procedure for supply and viscosity adjustment of oil for an oil damper which is in principle provided in order to restrict self-excitation and overshoot of the carburetor. To solve this drawback, there has been developed and actually installed an excellent variable venturi carburetor of an oil damper-less type having a gas-responsive mechanical design such as disclosed, for example, in the Applicant's previous Japanese Patent Application No. 94534/1978.
Even in the oil damper-less type variable venturi carburetor, a kind of limited fixed venturi construction is employed wherein a stopper is generally added to the barrel of the venturi section to avoid perfect closing of the suction piston in order to stabilize an air-fuel ratio in the range of a small air feed quantity occurring during the low negative pressure period from the time of low temperature cranking at the start of the engine till idling.
Consequently, it follows naturally that during the low temperature cranking or idling, the negative pressure does not rise up to a predetermined level and suction of the gasoline from the main nozzle becomes insufficient as much, thereby failing to provide an over-rich air-fuel ratio. To cope with this problem, a choke valve is therefore disposed on the upstream side of the venturi section to elevate the negative pressure while a starter nozzle is interposed between the choke valve and the suction piston, so as to increase the amount of gasoline when the choke valve is operated.
However, the abovementioned counter-measure increases inevitably the height of the carburetor by the height of the choke valve and thus leads to the problem in that the height of the engine room of a sport car or the like increases as much eventually. In addition to the complicated construction due to the provision of the starter nozzle, there occurs another problem such as insufficient power or unstable driving performance because low temperature acceleration and warming-up driving must be controlled by the single choke valve. If a control- or adjustment-mechanism is added so as to compensate for these drawbacks, there occur again such problems as a still further complicated construction, increase in the cost of production and troublesome maintenance.